Image enhancement is often used to improve image quality, and image sharpening is one way of achieving image enhancement.
Generally speaking, a conventional image sharpening method typically includes the following steps. Initially, a plurality of edge pixels in an image are detected. Subsequently, the grayscale intensity of the detected edge pixels and their surrounding pixels is changed to achieve the effect of enhancing image sharpness. Although such a method can enhance contrast between the detected edge pixels and their surrounding pixels, the grayscale intensity of the detected edge pixels and their surrounding pixels may be changed so much that the contrast between the detected edge pixels and their surrounding pixels is excessive, thereby resulting in the generation of clipping and ringing in the image. This may cause the human eyes to easily perceive unnatural over-sharpening in the image.
In order to overcome the over-sharpening drawback, U.S. Patent Application Publication No. US20050135701, for example, discloses a method including the following steps. First, a set of detected edges in an image is acquired. Subsequently, a set of exemplary edges is determined from the set of detected edges in the image. Thereafter, the degree of image sharpening is predicted based on the set of exemplary edges, and an average normalized overshoot percentage (hereinafter referred to as OS) is obtained accordingly. Finally, using the OS as a determination parameter, as set forth below, a determination is made as the manner to further improve the sharpness of the image.
  {                                                                                               If                  ⁢                                                                          ⁢                  O                  ⁢                                                                          ⁢                  S                                <                                  15                  ⁢                  %                                            ,                              perform                ⁢                                                                  ⁢                complete                ⁢                                                                  ⁢                sharpening                ⁢                                                                  ⁢                processing                                      ⁢                                                                                                                                                  If                  ⁢                                                                          ⁢                  15                  ⁢                  %                                ⩽                                  O                  ⁢                                                                          ⁢                  S                                <                                  25                  ⁢                  %                                            ,                              perform                ⁢                                                                  ⁢                partial                ⁢                                                                  ⁢                sharpening                ⁢                                                                  ⁢                processing                                      ⁢                                                                                                                                                  If                  ⁢                                                                          ⁢                  25                  ⁢                  %                                ⩽                                  O                  ⁢                                                                          ⁢                  S                                <                                  35                  ⁢                  %                                            ,                              perform                ⁢                                                                  ⁢                no                ⁢                                                                  ⁢                sharpening                ⁢                                                                  ⁢                processing                                      ⁢                                                                                                                                                  If                  ⁢                                                                          ⁢                  O                  ⁢                                                                          ⁢                  S                                ⩾                                  35                  ⁢                  %                                            ,                              perform                ⁢                                                                  ⁢                blurring                ⁢                                                                  ⁢                processing                                      ⁢                                                                    
If the OS value is large, this indicates that the degree of image sharpening is high. For instance, if OS≧35%, this indicates that the image has been over-sharpened and requires blurring or de-sharpening processing. On the other hand, if the OS value is small, this indicates that the degree of sharpening of the image is low or the image is blurry. For instance, if OS<15%, this indicates that the image has not been adequately sharpened and requires complete sharpening processing.
Since the OS value is used as the determination parameter, the degree of image sharpening can be limited so as to solve the problem of over-sharpening. However, the generation of ringing noise at the edges of the image still cannot be avoided after the aforesaid processing. Therefore, prior art apparently has room for improvement.